Typical well-known methods for forming micro- or nano-scale relief patterns by a roll-to-roll process generally involve the use of a rotary master pattern tool (“template”), usually in the form of a roll or belt, which contains the relief pattern to be replicated. The substrate having a replicated pattern can itself be the desired result, such as an optical pattern (diffractive optical elements, microlens arrays, optical security features, etc.) or a pre-formed surface upon which additional layers are formed (optical data storage preformat, embossed metallized hologram, micropixel array, microfluidic channels, etc.). Another application of relief patterns is in the formation of lithographic masks or stencils used to form patterned metallic or dielectric layers, such as for electronic circuits and the like.
In general, such relief patterns can be formed by any of several basic methods of molding, such as heat, pressure, chemical, and radiation curing, which can either be used alone or in various combinations. For roll-to-roll patterning, a deformable layer, typically in the form of a polymeric substrate, a multilayer substrate (coated or co-extruded layers on a carrier substrate) or a curable liquid layer, is continuously fed into a replication area of the roll processing machine. In the case of “dry” substrates, the application of heat and/or pressure causes the top surface or top layer(s) of the incoming film to soften and comply to the pattern of the patterning tool. Re-solidification, such as by cooling, etc., while in contact with the tool “sets” the pattern prior to separation of substrate and tool. For “liquid” processes, a fluid in the form of curable polymer or a liquid chemical is applied to the incoming substrate or patterning tool prior to contact between substrate and patterning tool. For radiation curable liquids, radiation of a suitable wavelength (typically generated by UV, visible, or electron beam sources, etc.) is applied while the liquid, substrate and tool are in simultaneous contact, and after solidification of the liquid the film is separated from the tool, its surface now being a replica of the tool pattern.
Other forms of pattern replication that do not utilize a molding process, such as photolithography, can also be carried out by roll-to-roll means. In this case, a photosensitive layer is applied to a carrier substrate and is exposed by radiation of an appropriate wavelength through a photomask containing transparent and opaque areas. The photosensitive layer is selectively exposed to the radiation passing through the transparent areas only, and a subsequent development or removal step is required to form the desired relief pattern. The exposure step can be carried out by projecting the exposing photomask image onto the photosensitive layer or by contact of the mask to the photosensitive layer during exposure. In the photomask process, diffractive effects can result in the exposed pattern being broadened or otherwise different from the pattern of the photomask, thus pattern resolution is a function of the pattern size and exposure wavelength.
Other methods for forming material patterns (i.e., patterns comprised of discontinuous areas of metals and/or dielectrics on a substrate) include coating or otherwise applying one or more material layers over a mask or stencil (additive process) or removal of one or more material layers through a mask or stencil (subtractive process). The additive step is typically carried out by blanket coating of the entire surface (such as by vacuum deposition) and the subtractive by immersion of entire substrate (wet etching), where the actual pattern is defined by the removal of the relief mask or stencil.
Many methods have been developed for applying liquid layers to substrates in roll-to-roll processes, including gravure/flexographic printing, reverse roll coating, drip coating, slot (die)-coating, wire-wound rod applicator, pressure nip, lamination, inkjet as well as other techniques. The liquids may be in the form of aqueous or solvent solutions, radiation-curable monomers (e-beam, UV, etc.), or chemicals that interact with the substrate or coating. Many of these application techniques have been developed by the graphic arts industry for applying inks and varnishes to substrates. However, although these techniques are adequate for general roll coating and printing, they all suffer from limitations in resolution, feature acuity, and ability to modify liquid deposition characteristics and locations “on the fly”, all of which are important in pattern replication.
Furthermore, most conventional liquid application processes used in pattern replication are not capable of forming the ultra-thin layers required for certain applications. One common method used to blanket coat thin film layers is to add a solvent or diluent to the fluid such that after application of the coating solution, removal of the diluent produces a thinner layer than the originally applied “wet” layer (in proportion to the solid/diluent ratio). However, this has several drawbacks: it requires a means to remove the diluent, e.g., a drying oven of suitable length, it is susceptible to particulate contamination, and it can be difficult to maintain precise coverage uniformity. The drying process, which typically uses hot air or inert gas, exposes the layer to the possibility of particulate contamination as well as layer disturbance due to air impingement and turbulence. Due to variations in wetting, surface flatness, etc., spreading and uniformity of the coated layer cannot be controlled to the degree required for replication of very high resolution patterns.
The precise control of thickness uniformity and spatial distribution of the applied liquids is particularly crucial for roll-to-roll patterning. In some cases, for example where nanoscale features are to be formed, it is very important that the fluid be applied with a very high degree of thickness uniformity and special precision, and in a manner that minimizes particulate contamination. Forming nanoscale features often requires extremely thin layers, and optimum pattern replication may also require modulating the coating thickness, viscosity or other physical or material parameters. These capabilities are not available with the current art.
Another area requiring high replication fidelity and pattern uniformity is the formation of relief masks for subtractive or additive processes, the formation of which typically produces a certain amount of polymer residual (“scum”) at the bottom of the formed features, whether by molding or conventional lithography. Because the residue layer covers the layer under the mask and blocks exposure to the desired effects of additive or subtractive processing, it must be removed (“de-scummed”) before further processing. This is typically done by plasma (“dry”) etching. However, thickness variations of the residue layer can cause serious problems for precise mask definition using such molded masks: if the residue thickness varies in different areas of the pattern, then the plasma etch process will result in insufficient residue removal in some areas (where residue is too thick) and excessive removal in others (where the residue is too thin). Insufficient etching will not clear (or not fully clear) the residue from the bottom of the mask, resulting in connected (or larger) features in subtractive processes, or missing (or smaller) features in additive processes. Overetching will remove too much material and make the features too small (subtractive processing) or too large (additive processing). Such pattern variations are unacceptable for precision and/or nanoscale applications.
In addition to lack of thickness or spatial uniformity of the applied polymer or fluid layer, another source of poor residue uniformity is the change in effective volume of the cavities of the patterning tool formed between the tool and the substrate. This results from hydrodynamic and other forces that can distort the substrate (and possibly the tool) when they are brought together under pressure and can result in areas in which the thickness of the mask itself varies, particularly in large areas that have no pattern or relatively large feature sizes. For example, if the mask to be formed is to cover a large unpatterned area (for example, many times the mask feature width), the separation distance between the tool and the substrate may be reduced in the center of this area, since substrate flexibility and the polymer fluid dynamics of the polymer can cause the two surfaces to pull together or spread apart, reducing or increasing, respectively, the thickness of the intervening polymer layer and the resulting mask feature height. During plasma removal of the residue layer, abnormally thin sections of the mask may be etched away, resulting in defects in the formed pattern. These problems are exacerbated with the thin polymer substrates typically used in roll-to-roll manufacturing.
There is a need for methods and systems by which the above shortcomings and limitations of the current coating and patterning art can be remedied.